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From the developer's news post on the matter:

This Halloween has claimed another victim, Afterthought Games. Yes, we are closing down forever. That means this is your last chance to buy Cornflower Corbin, or Unreal Estate before they are out of stores for good.

Valve has re-assured us that those whom have purchased the games and those that purchase the games before our studio closes will continue to be able to install and play them into the future.

From what the reviews for the games state, the online multiplayer function for Unreal Estate was both severely underdeveloped and completely empty to begin with, so even if that functionality does shut down [and if the game uses basic Steamworks connectivity for direct connections versus actual "servers", that won't necessarily happen] the single player / local multiplayer components should still function fine.

By my understanding Unreal Estate allows up to 4 players, of which up to 3 can be set to AI, meaning there should still be some good local functionality even if online connectivity becomes officially broken [versus just unofficially]. Meanwhile, Cornflower Corbin doesn't actually have any online functionality whatsoever [apparently the dev mistagged it, as their description appears to explicitly state that it's local muiltiplayer only], so there shouldn't be any change there whatsoever.

DIG has Unreal Estate for 20 cents.

In this case, achievements should work as intended. But if the game requires to connect to a game server, you won't be able to unlock them anymore. (RIP DI:E)